4.1 Introduction to CASTEP (1)

baconossifiedMechanics

Oct 29, 2013 (3 years and 9 months ago)

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1

4.1 Introduction to CASTEP (1)


CASTEP is a state
-
of
-
the
-
art quantum mechanics
-
based program
designed specifically for solid
-
state materials science. CASTEP
employs the density functional theory plane
-
wave pseudo
-
potential
method, which allows you to perform first
-
principles quantum
mechanics calculations that explore the properties of crystals and
surfaces in materials such as semiconductors, ceramics, metals,
minerals, and zeolites.


Typical applications involve studies of surface chemistry, structural
properties, band structure, density of states, and optical properties.
CASTEP can also be used to study the spatial distribution of the
charge density and wave functions of a system.

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4.1 Introduction to CASTEP (2)


CASTEP can be used effectively to study properties of both point defects
(vacancies, interstitials, and substitutional impurities) and extended defects
(e.g., grain boundaries and dislocations) in semiconductors and other
materials.


Furthermore, the vibrational properties of solids (phonon dispersion, total
and projected density of phonon states, thermodynamic properties) can be
calculated with CASTEP using either the linear response methodology or
the finite displacements technique. The results can be used in various ways,
for instance, to investigate the vibrational properties of adsorbates on
surfaces, to interpret experimental neutron spectroscopy data or vibrational
spectra, to study phase stability at high temperatures and pressures, etc. The
linear response method can also be used to calculate the response of a
material to an applied electric field
-

polarizability for molecules and
dielectric permittivity in solids
-

and to predict IR spectra.

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4.2 Modeling

Figure 4.1. Model for the motion energy calculation.

(a)
Supercell with an extra oxygen vacancy.

(b)
Supercell with a oxygen ion located at the saddle point.

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4. 3 Choosing Parameters



The structural parameters were calculated by geometry
optimization

with high accuracy from first principles.


Calculations of the total energy of the systems were carried out
using the pseudo
-
potential method.


All static calculations were performed using the CASTEP code with
ultra soft pseudo
-
potentials and the
Perdew
-
Burke
-
Ernzerhof

(PBE)
GGA exchange correlation term.


The ultra
-
find convergence of the total energy and the energy cutoff
were chose.


Spin polarization was taken into account in this study.

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4. 4 Examples


Example 1: migration energy of oxygen ion in the
BaCo
0.875
B
0.125
O
3
(
B
=

Sc, Mn, Ni, Fe, Co, Y, Nb, In, Sn
)

Figure 4.2 Migration energy of oxygen ion in the

BaCo
0.875
B
0.125
O
3

system

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4. 4 Examples


Example 2:migration energy of oxygen ion in the
Ba
0.875
A
0.125
CoO
3
(
A
=

La, Sr, Ba, Ca
)

Figure 4.3 Migration energy of oxygen ion in the

Ba
0.875
A
0.125
CoO
3

system

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4. 4 Examples


Example 3: Calculation for oxygen ion migration in
SrBO
3


The aim of calculation was to choose a better element as
a B
-
site dopant in SrTiO
3

to lower the oxygen ion
migration energy and thus increase the oxygen ion
conductivity. The ion migration energy was referred to as
the activation energy for oxygen ion conduction. The ion
migration energies in SrBO
3

(B=Sc, Ti, V, Cr, Mn, Fe,
Co, Ni, Zn, Ga, Ge, As, Zr, Nb, Mo, Tc, Ru, Rh, Pd, Cd,
In, Sn and Sb)

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Figure 4.4 Migration energy of oxygen ion in the

SrBO
3

system

X. Li et al. / Electrochemistry Communications 10 (2008) 1567

1570

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4.5 Testing Methods


Electron
-
blocking method


Thermogravimetric testing


Concentration cell


Chemical titration